In cold weather, certain helicopters start and operate poorly, particularly certain critical components or systems. These systems include the engine gear boxes, the main rotor gear case or gearbox, the intermediate gear case, the tail rotor, the oil tank and the oil cooler assembly, and on certain engines, the fuel control unit and drive reduction gear case. The gear cases have a poor fit about the gears due to the differential rates of expansion of the various metals of which they are made.
For example, gearboxes are constructed of aluminum while the gears are steel, which has half the coefficient of thermal expansion. The unit is assembled at room temperature and designed to withstand high temperatures. At low temperature, the aluminum contracts at twice the rate of steel, resulting in a tight gear fit. Magnesium gear cases have an even higher coefficient of thermal expansion. For proper fit, they must be brought up to a warmer temperature. A turboshaft engine will usually start in cold weather. However, due to poor component fits at low temperature, the engine may be slow to accelerate. The slow acceleration will cause higher temperatures in the burner cans and turbine section of the engine. This is called a "hot start". This will at best shorten the engine life and, at worst, destroy the hot section of the engine. Another problem exists in the reduction gearing to the power shaft of the engine. This area of the engine is subject to abnormal wear and sometimes failure due to the differential rates of expansion of the components. The fit of bearings and oil transfer sleeves is sometimes incorrect at low temperatures.
The batteries on board the helicopter have less energy available for starting the engines when cold. At low temperatures, the cold turboshaft engine may require more power to start than the cold batteries can deliver. For this reason, it is desirable to warm the turboshaft engines of the helicopter. The oil tank and oil cooler can be preheated so the oil will maintain uniform heat transfer throughout the various components of the helicopter. The oil also provides lubrication. Turbine oil used in the engine flows well at low temperatures, but the heavier gear lubricating oils used in transmissions need warming to lubricate properly.
One method of bringing the helicopter systems up to temperature is to run the engine for 15 to 20 minutes in moderately cold weather. Often helicopters are used for medical rescue or other applications in which quick response is needed. These aircraft are often based at locations where conventional preheat systems cannot be used.
Some conventional systems include gas-fired, hot air blowing preheaters. These require partial dismantling of the aircraft and preheat only one piece of equipment at the time. In certain locations, they are not legal for use.
Electric automotive-type preheaters with resistant elements and blower fans have been used. These require partial disassembly of the aircraft for use. The units have been known to present a fire danger.